Breakdown of resistance to sorghum midge, Stenodiplosis sorghicola

Sorghum midge (Stenodiplosis sorghicola Coquillett) is an important pest of grain sorghum worldwide. Several sources of resistance to sorghum midge ha ve been identified in the world sorghum germplasm collection, of which some lines show a susceptible reaction in Kenya. Therefore, we studied the inse ct density damage relationships for a diverse array of midge-resistant and midge-susceptible sorghum genotypes, and variation in association of glume and grain characteristics with expression of resistance to sorghum midge. A F 28 and IS 8891 showed resistance to sorghum midge both in India and Kenya ; DJ 6514 and ICSV 197, which are highly resistant to sorghum midge in Indi a, showed a susceptible reaction at Alupe, Kenya. Sorghum midge damage in g eneral was greater in Kenya than that observed in India at the same level o f midge density suggesting that the breakdown of resistance in Kenya is due to factors other than insect density. Glume length, glume breadth, and glu me area were positively associated with susceptibility to sorghum midge at both locations. However, under natural infestation, the correlation coeffic ients were stronger in India than in Kenya. Grain mass at 3 and 6 days afte r anthesis was positively associated with susceptibility to midge in India, but did not show any association with midge damage in Kenya. Grain growth rate between 3 and 6 days after anthesis was more strongly correlated with susceptibility to midge in Kenya than in India. Variation in the reaction o f sorghum genotypes across locations may be partly due to the influence of environment on association between glume and grain characteristics with sus ceptibility to sorghum midge, in addition to the possible differences in mi dge populations in different geographical regions.